The invention relates to improvements in paper making machines and more particularly to a stock screen for screening the stock prior to its entering the headbox wherein the pulsations in the stock caused by the rotating foils are damped to prevent the pulsations from causing disturbances in the headbox flow.
In a paper making machine, stock is delivered under pressure to a headbox from which it is discharged onto a traveling foraminous surface for dewatering and web formation. It is essential and critical to uniform operation and good formation of the web that pulsations and pressure in the headbox be avoided. One source of such pulsations occurs in the screening chamber through which the stock flows prior to delivery to the headbox for the purpose of cleaning the stock and removing undesirable foreign particles such as dirt, bark, slivers and other material. Such screening chambers conventionally have a cylindrically shaped screen through which the stock flows, and in order to maintain flow through the screen and prevent the screen from being clogged, one or more foils are continuously passed at uniform speed over the surface of the screen to cause a pressure pulsation. While such pressure pulsation is necessary to the satisfactory operation of the screen, it introduces a pressure pulsation which is felt in the stock delivery to the headbox. Such pressure pulsations will disturb uniform distribution of the stock and will have an adverse effect on flow of the stock through the slice opening onto the forming surface, and it is essential to satisfactory high speed operation and good formation that the fluctuations be essentially eliminated.
It is accordingly an object of the present invention to provide an improved paper making machine with a stock screening device in the flow supply to the headbox which does not emit pressure fluctuations and pulsations that would be deleterious to good stock distribution and flow through the headbox.
A still further object of the invention is to provide an efficient effective paper stock screening apparatus using a cylindrical screen with a rotating foil wherein pressure fluctuations inherent with the operation of the foil are effectively damped and eliminated.
A further general object of the invention is to provide an improvement in a paper making machine having screening apparatus with an improved screening structure.
Other objects, advantages and features, as well as equivalent structures and methods which are intended to be covered herein, will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: